In boots of this type, which are generally made of a molded plastic material, the shaft may be comprised of several parts: a collar and a rear flap may be jointed o the shell-base along a transverse axis. If required for a particular shoe, the joint axis may not be given physical form, but the shaft must be able to bend, at least to some degree, in relation to the shell-base, according to the conditions in which the boot is used; this arrangement is the equivalent of a joint.
The general direction of the shaft, which is tightly fastened to the shell-base, is called "the shaft axis" for the sake of simplicity, and inclines anteriorly in relation to the vertical at a particular angle, called the "angle of projection." This angle of projection is liable to vary around a median value, according to the circumstances existing at the time of use, as a result of the joining of the shaft to the shell-base. As a general rule, the taller the skier and the more the skier skis in an extremely flexed position, the more he will desire a high median (static) angle of projection; this angle may be on the order of 13.degree. to 20.degree., and may occasionally even reach 25.degree., while, for the tourist skier, an angle of 8.degree. to 15.degree. is generally considered optimal. Furthermore, the skier will, according to the type of skiing being practiced and to the snow conditions, want more or less flexibility in the joint attaching the shaft to the shell-base, i.e., greater or lesser ease of achieving variation of the angle of projection as a result of the force exerted. Thus, rather pronounced stiffness is desired for competition, on packed snow or ice, while more flexibility is desired on powdery snow in order to gauge better the points of support, and during descent in order to use muscular effort economically and to seek a flat ski position.
What these considerations mean is that the ever-increasing number of skiers feel the need for boots that not only have a median angle of projection corresponding to their personal criteria, but in which the stiffness of the joint attaching the shaft to the shell-base is in direct relation to the force exerted on the shaft, thus enhancing the users' comfort and safety.
Attempts have, therefore, been made to fulfill these requirements by means of devices which control this power of flexion. Many ideas have been advanced, e.g., boots described in French Patent Application No. FR 2 583 271 or in French Patent No. FR 2 569 088, which represent distinct progress over the previously-known technology. In these documents, use is made of flexion devices in the form of double elastic girders, which are attached to the shell-base, and of which at least a portion cooperates, by means of movable contact, with a surface or guide-ramp which acts as a cam, and which is carried on the shaft of the boot. In the first case, the double flexion girder is positioned in a generally transverse direction in relation to the boot. In the second, it is positioned in a generally longitudinal direction.
These two mechanisms are perfectly satisfactory in controlling stiffness of the shaft as it pivots in an anterior direction in relation to the shell-base, but they make no provision for the stiffness of the joint in the reverse direction of pivoting, that is, toward the rear when the skier requires posterior support. In the absence of any control, this rearward flexion may seriously compromise the safety and comfort of the skier, especially in very difficult conditions, as, for example, in competition. It is therefore essential to have mechanisms available which permit control of the pivoting of the shaft, both anteriorly and posteriorly in relation to the shell-base.
The present invention concerns a mechanism enabling bi-directional control.